Arsenic is a commonly occurring toxic metal in natural ecosystems. Its presence in natural waters may originate from geochemical reactions, industrial waste discharges, or agricultural use of pesticides containing arsenic (Gupta, S. K. and Chen, K. Y., "Arsenic Removal by Adsorption," 50(3) Journal of Water Pollution Control Federation 493 (March. 1978)). Arsenic is also present in the gold cyanidation process. It is present in gold extraction processes which utilize roasting, pressure autoclaving and other oxidation pathways. The presence of arsenic in water beyond the currently permissible limit of 50 ppb has carcinogenic effects on living things (Shen, Y. S. and Chen, C. S., Proceedings of 2.sup.nd International Congress on Water Pollution Resources, Tokyo, Vol. 1, Pergamon Press, New York, 173-179 (1964)). Hyperpigmentation, skin cancer, liver cancer, circulatory disorders and other ailments have been attributed to the presence of arsenic in water (National Academy of Sciences, "Arsenic--Medical and Biological Effects of Environmental Pollutants," U.S. Government Printing Office, Washington, D.C. (1977)). Although it has been suggested that arsenic may be an essential micro nutrient for humans and animals (Krapf, N.E., "Commercial Scale Removal of Arsenite, Arsenate and Methane Arsenate from Ground and Surface Water; Arsenic: Industrial, Biomedical, Environmental Perspectives," W. Lederer and R. Fensterheim (eds.), Van Nostrand Reinhold Company, New York, N.Y., 269 (1983)), the presence of arsenic in amounts exceeding suggested nutritional requirements can be detrimental.
In the United States, arsenic concentrations in waste waters, surface and ground waters, and geothermal waters frequently exceed the recommended limits for drinking water (United States Public Health Service Drinking Water Standards, U.S. Department of Health, Education and Welfare, USPHS Publication No. 956, (1962)). The United States Environmental Protection Agency is currently re-evaluating the existing Maximum Contaminant Level (MCL) for arsenic in drinking waters which is currently 0.05 mg/l. The revised MCL for arsenic is expected to be somewhere between 0.002 and 0.02 mg/l (Pontius, F. W., "Crafting a New Arsenic Rule," 86 Journal of American Water Works Association, 6-10 (1994)).
Arsenic occurs in inorganic form in aquatic environments, resulting from the dissolution of solid phases such as arsenolite (As.sub.2 O.sub.3), arsenic anhydride (As.sub.2 O.sub.5) and realgar (AsS.sub.2). The chemistry of arsenic in aquatic systems is complex because the element can be stable in four major oxidation states (+5, +3, 0 and -3) under different redox conditions. In natural waters arsenic is found as an anion with acid characteristics in only the As(III) and As(V) oxidation states. In oxygenated waters, the oxyanions of arsenic exist in four different arsenate species as H.sub.3 AsO.sub.4, H.sub.2 AsO.sub.4.sup.-, H.sub.3 AsO.sub.4.sup.2- and AsO.sub.4.sup.3- in the pH ranges of &lt;2, 3-6, 8-10 and &gt;12, respectively. Under mildly reducing conditions the arsenite species H.sub.3 AsO.sub.3, H.sub.2 AsO.sub.3.sup.- and HAsO.sub.3.sup.2- become predominant in the pH ranges of 7-8, 10-11 and 12-13, respectively (Wasay, S. A., Haron, Md. J. and Uchiumi Akira, "Removal of Arsenite and Arsenate Ions from Aqueous Solution by Basic Yttrium Carbonate" 30 Water Resources, 1143-1148 (1995)). The amounts of the various species of arsenic and the stability of the various species in a given solution depends on the pH of the solution (Gulledge, J. H. and O'Connor, J. T., "Removal of Arsenic (V) from Water by Adsorption on Aluminum and Ferric Hydroxides" 65 Journal of American Water Works Association, 548-552 (1973)). In most waters, a mixture of As(III) and As(V) species are usually present.
Selenium also occurs in inorganic form in aquatic environments. Selenium typically occurs in the selenate (VI) and selenite (IV) oxidation states. The presence of selenium in drinking and ground water also causes health and environmental problems similar to the problems that exist when arsenic is present. The current allowable maximum concentration level for selenium in drinking water set by federal standards is 0.01 milligrams per liter (Baldwin, R. A., et. al. "Process for the removal of selenium from aqueous solution," U.S. Pat. No. 4,405,464).
Several methods for reducing selenium and arsenic concentrations to acceptable levels have been studied and are being used currently. These methods include coagulation and precipitation using ferric chloride and sulfate, ion exchange, reverse osmosis and adsorption using activated carbon and alumina. These methods are effective to a certain extent. However, these methods are considerably more expensive and generally narrower in application than is desired for the treatment of large volumes of water.
The use of ferric chloride, hydrated lime, sodium sulfate and alum to coagulate water containing arsenic has been described (Harper, T. R. and Kingham, N. W. "Removal of arsenic from wastewater using chemical precipitation methods," 64(3) Water Environment Research 200-203 (1992)). Ferric chloride has also been used to precipitate selenium. These methods require multiple treatments of the water with the coagulation chemicals and large amounts of chemicals relative to the amount of arsenic and selenium present to obtain the desired reduction in arsenic concentration. In addition, the methods produce sludge that requires dewatering or solidification and eventual landfill storage as hazardous waste. Also, the ferric chloride process requires pH of less than 6.5 (Merrill, D. T. et al., "Field Evaluation of Arsenic and Selenium Removal by Iron Coprecipitation," 6(2) Environmental Progress 82-90 (1987)).
In addition to the waste disposal problems and large amounts of reagent chemicals required, laboratory tests and pilot plant studies have shown that chemical precipitation employing alum, lime, ferrous sulfate or ferric sulfate, is substantially ineffective for removing selenium in the selenate (Se(VI)) oxidation state from water. Studies on water having a selenium Se(VI) concentration of 0.03 to 10 milligrams per liter have shown that the conventional chemical precipitation methods remove less than ten percent of the selenium from the water (U.S. Environmental Protection Agency, "Manual of Treatment Techniques for Meeting the Interim Primary Drinking Water Regulations," 29-31 (May 1977)).
U.S. Pat. No. 4,405,464 provides a method by which selenium, as selenate, can be chemically precipitated from an aqueous solution using metallic iron. The patent also discloses removal of a substantial portion of selenium in its selenite oxidation state. This process is economically more attractive than either the ion exchange or reverse osmosis methods which have been proposed or which are currently in use. However, this method is not suitable for aqueous solutions having pH greater than 6.0. Thus, if the water is alkaline or neutral, it must be acidified through the addition of an acid. Also, the method does not reduce the selenium concentration in water to meet drinking water quality standards.
U.S. Pat. No.3,933,635 discloses a process for removing selenium ions present in the selenite oxidation state from acidic process waters. Acidic process water, having a pH of about 1.0 to 4.0, is reacted with a metallic reducing agent at a temperature in the range of about 25.degree. C. to about 85.degree. C. for a sufficient time to reduce the soluble selenium in the selenite oxidation state to insoluble elemental selenium. Preferably, the temperature is maintained in the range of about 50.degree. C. to about 70.degree. C. The reducing agent can comprise aluminum, iron or zinc in an appropriate form, such as, for example, powders, scrap fragments, granules, wools and the like. The preferred reducing agent for selenium in the selenite oxidation state is zinc powder.
U.S. Pat. No. 4,519,913 provides a method of reducing the concentration of water-soluble ionic selenium species in aqueous solutions through bacterial treatment in a porous matrix. While this method is disclosed as being effective for removing selenium ions in aqueous solution having pH between 6 and 11, the method requires the use of a specific bacterial strain and nutrients for the bacteria, and produces selenium metal in the porous matrix, which causes disposal problems. In addition, the flow rate of water through the porous membrane must be carefully controlled. The process also takes many clays of operation to cause the selenium concentration in the water to be reduced to acceptable levels.
A method of precipitating arsenite and arsenate ions from aqueous solutions using yttrium carbonate at alkaline pH has also been described (Wasay, S. A., et. al. "Removal of Arsenite and Arsenate Ions from Aqueous Solution by Basic Yttrium Carbonate" 30(5) Wat. Res. 1143-1148 (1996)). This method requires strict control of pH to achieve removal sufficient to comply with environmental standards. In addition, the effective pH range was found to depend on which arsenic species is desired to be precipitated.
U.S. Pat. No. 3,956,118 discloses a process for removing phosphate ions from waste waters using a rare earth salt. However, the disclosed process is limited to removal of phosphates.
Recently new adsorbents, lanthanum oxide and lanthanum-alumina oxide, have been used for removing arsenate, arsenite, selenate, and selenite species from solution (U.S. Pat. No. 5,603,838 (1997)). This patent discloses that lanthanum oxide alone or in conjunction with alumina solids and other oxides can remove arsenic and selenium to low levels (&lt;50 ppb). Also, the adsorption kinetics were found to be extremely fast compared to other adsorbents such as alumina.
The major drawbacks of the processes discussed above include inefficient removal of arsenic and selenium to an acceptably low level for drinking water and discharge into ground water, the problem of filtration of precipitated sludge and fouling of resins and membranes. In addition, once the arsenic and selenium species are removed, the solid materials formed must be disposed of The solid materials formed from the processes above are also susceptible to leaching of the metals at a future time.
There is a need to find a water soluble, highly selective and inexpensive reagent which effectively precipitates and stabilizes arsenate, arsenite, selenate and selenite over a wide pH range, present in solutions or solid/liquid mixtures.